class B tanks carried fuel oil and diesel and even pitch. the important point is the flash point of the liquid there were different regulations for the handling and loading. For a long time class A were not allowed bottom discharge due the the danger of leakage, More modern valves have now obviated this. Class A were (after a very early free for all) required to be silver grey or stone colour to reduce evaporation and aid identification likewise the red stripe (but this was dispensed with in wartime)
Class B liquids are more dense, so usually the tanks are slightly smaller (part filled tanks are bad news) however since the thicker liquids (fuel oil, pitch) often required to be warm to achieve reasonable loading/unloading times the tanks were often insulated so looked the same size. the really thick tar & pitch had to have stem heating coils fitted as well.
The converted air ministry tanks (Lionheart) may have just have had the max load upgraded. The air ministry tanks did not have vac brakes and there were rules about how the trains could be formed , usually one or more vans next to loco. The later tank as modelled by Heljan were higher capacity 20-22 tonnes and vac braked, These were themselves made obsolescent by later 35 40 45 ton air braked stock. Thus three distinct generations un braked, vac braked and air braked, that would not be mixed (in service). They also needed to match the loading unloading facilities.